There is a desire in the industry for higher circuit density in microelectronic devices which are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. The use of shorter wavelength radiation than the currently employed mid-ultraviolet spectral range (e.g., 248-450 nm) offers the potential for higher resolution. Ultra-deep ultraviolet radiation, particularly, 193 nm, has been proposed. See, for example, Allen et al. (1995), "Resolution and Etch Resistance of a Family of 193 nm Positive Resists," J. Photopolym. Sci. and Tech. 8(4):623-636, and Abe et al. (1995), "Study of ArF Resistant Material in Terms of Transparency and Dry Etch Resistance," J. Photopolym. Sci. and Tech. 8(4):637-642. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, current lithographic photoresist compositions (also sometimes termed "resist compositions" or "resists") absorb strongly at 193 nm as a result of their aromaticity, making them unsuitable for commercial use at 193 nm.
Acrylate and methacrylate resists are known in the art. For example, U.S. Pat. No. 5,071,730 to Allen et al. discloses a resist comprising a terpolymer of methyl methacrylate, t-butyl methacrylate and methacrylic acid. Allen teaches that the t-butyl group imparts acid sensitivity, the acid group influences development rate, and the methyl ester increases toughness of the film. Although this resist is suitable for some commercial uses, it is deficient in that it has low reactive ion etch resistance. Reactive ion etch resistance is desired for processing in semiconductor manufacturing. Further, excessive amounts of acid in the polymer will result in thinning in the unexposed areas during development.
Nozaki et al. (1993), J. Photopolym. Sci. Technol. 9:509, discloses a methacrylate resist having two acid-cleavable ester groups, adamantyl and pyranyl groups. The authors state that the resist develops well probably due to the high acid content of the exposed areas resulting from the cleavage of both leaving groups. However, the unexposed areas of the resist will experience substantial cracking during postexposure bake due to the large shrinkage in the exposed area upon removal of the large ester groups.
The requirements for photoresists for semiconductor manufacturing include etch resistance, clean aqueous development and compatibility with strong developers currently in use in semiconductor manufacturing. Traditional DUV resists (248 nm) combine these functions quite easily by using poly(hydroxystyrene), a phenolic resin, as the polymer of choice. This material has unacceptably high optical density for 193-nm lithography. Combining these requirements in a polymer that is nearly transparent at both 193 nm and 248 nm is a significant challenge.